Whatever else you can say about Skyfish's Soleil series, they never do the same thing (precisely) twice in a row.

The Soleil series is based in a multiverse that comes into existence after the end of Ragnarok, the final battle of the gods from Norse mythology.  Previous entries in the series have ranged from your typical old-style chuunige (Shirogane no Soleil) to crazed ventures into chaotic medlies of Norse, Chinese, and Cthulhu Mythos (Gouen no Soleil).  

Blade x Bullet is perhaps the  most straight-out 'after-story' of Norse mythology in the series yet.

Based in a world that is pretty much identical to our own (in fact, the US president in the setting is pretty familiar, lol), a decade or so before the beginning of the story, monsters called Gears, the undead remnants of the ancient deities, giants, and monsters killed in Ragnarok, have pretty much exterminated humanity.  Artificial Valkyries, made from weapons that have tasted the blood of many, the flesh of a mortal woman, and the soul of a Valkyrie, are the only weapon said to be capable of taking them on... but most nations failed to make them until it was too late (the US being the only exception, as their president was a crazy who took one look at the tech and put massive amounts of money into it, eventually making his daughter into one).  The protagonist, Yagi Mutsuki, is one of the rare humans who can and have fought the Gears on his own and taken them down, and at the beginning of the story, he dies and drops into Hel (Norse underworld) where the goddess of the same name informs him that he is the vessel of her father Loki's soul, and he wants him to join in a competition to determine which Valkyrie will become the god of the new world... and ensure that humanity's hope is cut off forever.  He then uses his dead lover's hair and an iron maiden (yes, the torture device) to make his Valkyrie, Reginleif (a Valkyrie who almost always seems to draw the short straw no matter what entry in the series she appears in).

Yagi Mutsuki is... pretty much what you would expect from the mortal incarnation of the Norse god Loki.  His essential nature is definitely chaotic, leaning toward evil.  He is something of a sadist, a schemer, and a trickster.  If there is a direct path and a twisty curvy path to the same goal, he will almost always take the twisty-curvy path.  He is also a blood-hungry warrior, almost a berserker.  Yet, it is surprisingly fun to watch him navigate the darkness of the competition.  He is capable of a limited sort of compassion... but he himself is never quite sure what he is feeling during those times when he shows it.

I'm going to be blunt... this is the most graphic game in the series, and while this series isn't known for being as extreme as stuff like Venus Blood, it definitely has its guro moments.  For those with a weak stomach or an aversion to blood, I recommend you find something else to play.  In addition, this is a game where the 'good guys' are actually worse than the natural villains, so I don't recommend this to people who like black and white stories or have problems when heroines happen to be serial killers or psychopaths.

However, it is also one of the most satisfying entries in the series, not the least because it doesn't waste time on separate routes in a game that has such a central plot.  The central story is there, it is interesting (if not godly), and it is a fun read, even with the more distasteful elements such as torture and SM.  Mutsuki's relationship with Reginleiv is so hilarious I often found myself giggling during serious scenes.  

In addition, there were some elements I found fascinating... such as Loki's punishment for his role in Ragnarok (I won't spoil it for you, but it is an interesting one), and how they integrated the various elements of the mythos into the flow of the story.   

While I don't consider this to be the best game in the series (I have a strong fondness for Gouen no Soleil), it is the easiest to pick up, because it isn't long enough to cause brain fry, but it is interesting enough that you won't feel like you wasted your time.